The switching power supply with characteristics of small, light weight and high efficiency is widely used in various types of electronic equipments, and it has become indispensable. The switching power supply is divided to an AC switching power supply and a DC switching power supply. The DC switching power supply is used to transforms the original eco-power with poor quality (heavy electricity) to the DC voltage with high quality to be meet the requirement of the device (light electricity). It should be understood that the DC switching power supply mainly includes a DC/DC conventer and a transistor component. The DC/DC conventer is used to output a pulse signal, such as a square wave signal, so as to control a time rate for turning on or turning off the transistor component. Since the DC/DC conventer outputs the pulse signal with the high frequency, it may cause that the electromagnetic interference (EMI) appears when an output level varies from a low level to a high level, as shown by (A) in FIG. 1. In order to suppress EMI, an output terminal may be connected to a ground through a RC snubber, so as to filter a noise generated by EMI, as shown in FIG. 2.
It should be further understood that a center frequency of the RC snubber is related to a resistor R and a capacitor C and is unchanged, and an oscillating frequency of the noise is related to a magnitude of a load current and is varied dynamically. It should be noted that the center frequency of the RC snubber is equal to or similar to the oscillating frequency of the noise. It may effectively filter the noise, so as to achieve the best efficiency for suppressing EMI. As shown by (B) and (C) in FIG. 1, when a center frequency corresponding R1 and C1 equals to an oscillating frequency 2 and a center frequency corresponding R2 and C2 equals to an oscillating frequency 3, it may respectively achieve the best efficiency for suppressing EMI. However, the existing RC snubber can not dynamically adjust the center frequency of a filtering, such that the efficiency for suppressing EMI is worse.